The present invention relates to a bimetallic catalyst and a process for the direct synthesis of hydrogen peroxide from hydrogen and oxygen which uses said catalyst.
Hydrogen peroxide is a commercially important compound which is widely used as bleach in the textile and paper industry, as a biocide in the environmental field and in oxidation processes in the chemical industry.
Examples of these processes are those using titanium silicalite as catalysts, such as the epoxidation of olefins (EP-100,119), the ammoximation of carbonyl compounds (U.S. Pat. No. 4,794,198), the oxidation of ammonia to hydroxylamine (U.S. Pat. No. 5,320,819) and the hydroxylation of aromatic hydrocarbons (U.S. Pat. No. 4,369,783).
The industrial production of aqueous solutions of H2O2 by means of a complex two-step process, is known. In this process a solution of an anthraquinone, such as butylanthraquinone or ethylanthraquinone, in an organic medium immiscible with water is first hydrogenated and then oxidized with air to produce H2O2 which is subsequently extracted in aqueous phase.
This procedure is expensive owing to the high investment costs necessary for the complex production unit involved and the necessity of separating and disposing of the by-products generated during the oxidation phase and purifying and reintegrating the anthraquinone solution before being re-used.
Processes for the direct synthesis of hydrogen peroxide from H2 and O2 have been proposed in the art for over-coming these disadvantages. These processes generally use a catalytic system consisting of a noble metal, particularly metals of the platinum group or their mixtures, in the form of salts or as supported metals.
For example, U.S. Pat. Nos. 4,772,458 and 4,832,938 describe a process for the synthesis of aqueous solutions of H2O2 using a catalyst based on palladium and/or platinum supported on carbon, in the presence of hydrogen ions and bromide ions. Quantities of acids ranging from 2.5 to 10 g/liter are generally required for obtaining high concentrations of H2O2.
The use of high quantities of acids creates serious problems linked to the dissolution of the active phase (metal) of the catalyst in the reaction medium, with the consequent instability of both the catalyst and hydrogen peroxide solution produced.
In addition, under these conditions, the solutions of H2O2 produced are difficult to use owing to the high acid content. These processes also operate under critical conditions as concentrations of H2 higher than 5% (17% or over) are used with respect to the reaction mixture and consequently fall within the explosivity limit of H2/O2 mixtures.
Patent application EP-492.064 describes a process for the synthesis of hydrogen peroxide from hydrogen and oxygen which uses a catalyst based on palladium supported on a halogenated resin, in particular a brominated styrene/divinylbenzene resin.
The reaction is carried out in water, in the presence of an acid selected from sulfuric, phosphoric or nitric acid. Operating according to this process however concentrations of H2O2 of about 0.58% are obtained.
Patent application EP-504,741 describes a process for the synthesis of H2O2 from hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on an acid or superacid carrier selected from oxides of molybdenum, zirconium or tungsten.
Operating according to this process, hydrogen peroxide is obtained in concentrations of not more than 1%.
U.S. Pat. No. 5,320,921 describes a process for the synthesis of H2O2 from hydrogen and oxygen which uses a catalyst based on palladium or platinum supported on a heteropolyacid made insoluble in water. The reaction is carried out in water in the presence of bromide ions (0.5 mmoles/liter of sodium bromide). Concentrations of H2O2 equal to about 1.1% are obtained.
These processes of the known art are consequently characterized by a low productivity and selectivity of the reaction and by the production of H2O2 solutions which are too dilute for an economic industrial exploitation.
Italian patent application MI 98-A-01843, filed by the Applicant, relates to a process for the direct synthesis of hydrogen peroxide using a catalyst based on palladium and/or platinum co-impregnated on a sulfonated activated carbon.
It has now been found that by carrying out the reaction between hydrogen and oxygen in the presence of a bimetallic catalyst prepared by dispersing in sequence and alternating the precursors of the single metal components of the catalyst on a carrier, it is possible to overcome the drawbacks of the known art described above. This preparation method also allows the activity and selectivity of the catalyst, the concentration of hydrogen peroxide or consumption of hydrogen, to be modulated in relation to the process requirements and availability of raw materials, for example low cost hydrogen.
In particular, the use of these catalysts gives the following advantages:
(i) the possibility of using extremely low quantities of halides ( less than 10xe2x88x924 moles/l) and free acids (H+ less than 10xe2x88x922 moles/l) in the reaction medium. This has beneficial effects on the stability of the catalytic system and H2O2 solutions obtained, thus allowing the possibility of directly using the above solutions in oxidation processes. The presence of only traces of halides and acidity is not a disadvantage for direct use in the above reactions as the possible introduction of salts or acidity in downstream processes is minimized.
(ii) the production of hydrogen peroxide solutions in adequate concentrations for direct use and economically valid in oxidation processes, generally ranging from 2% to 10% by weight.
(iii) the possibility of carrying out the reaction under high safety conditions. In fact, below 4% by volume of hydrogen means operating well outside the explosivity range of inert H2xe2x80x94O2 mixtures.
In accordance with this, the present invention relates to a bimetallic catalyst obtained by dispersing in sequence and alternating the precursors of the single metal components of the catalyst on a carrier, and a process for the synthesis of hydrogen peroxide by the direct reaction of hydrogen with oxygen, in a solvent medium containing a halogenated promoter and an acid promoter, in the presence of said catalyst.
In particular, the catalyst used for the purposes of the present invention is obtained by:
(a) preparation of the solutions or suspensions of precursors of the single metal components of the catalytic system;
(b) dispersion of the solutions or suspensions obtained in (a) in sequence on a carrier;
(c) treatment of the catalyst with a reducing agent of the metal and drying at 120-140xc2x0 C. between one dispersion and another.
In the preparation of said catalyst, steps (b) and (c) can be repeated once or several times.
The metal components of the catalyst are selected from those of the platinum group. Palladium and platinum are preferably used.
In these catalysts the palladium is normally present in a quantity ranging from 0.01 to 5% by weight and the platinum in a quantity ranging from 0.01 to 1% by weight, with an atomic ratio between platinum and palladium ranging from 0.1/99.9 to 50/50.
The palladium is preferably present in a quantity ranging from 0.4 to 2% by weight and the platinum in a quantity ranging from 0.05 to 0.5% by weight, with an atomic ratio between platinum and palladium ranging from 1/99 to 30/70.
The dispersion of the active components on the carrier can be effected by means of precipitation, impregnation or adsorption starting from solutions of their salts or soluble complexes selected from acetates, halides and nitrates.
The reduction of the catalyst components to the metal state can be carried out by means of thermal and/or chemical treatment with reducing substances such as hydrogen, sodium formiate, sodium citrate, using preparative methods well known in the art.
The inert carrier can consist of activated carbon, silica, alumina, silica-alumina, zeolites and other materials well known in the art. Activated carbon is preferred for the preparation of the catalysts useful for the invention.
Activated carbons which can be used for the purposes of the invention are those with a low ash content and a surface area of at least 100 m2/g, in particular those having a surface area greater than 300 m2/g.
Sulfonated activated carbons described in Italian patent application MI 98-A-01843 can also be used for the purpose.
The carriers can be in powder, grain or pellet form, etc.
Before supporting the precursors of the single metals, the activated carbon can be subjected to treatment such as washing with distilled water or treatment with acids, bases or diluted oxidizing agents, for example acetic acid, hydrochloric acid, sodium carbonate and hydrogen peroxide.
In particular, it has been observed that the catalyst obtained by dispersing first the palladium and then the platinum on the carrier, is more active, whereas that obtained by dispersing first the platinum and then the palladium in sequence, is more selective.
The catalyst of the present invention is particularly advantageous in a process for the direct synthesis of hydrogen peroxide from hydrogen and oxygen in a solvent in the presence of a halogenated promoter and an acid promoter.
The catalyst is used in catalytic quantities generally ranging from 10xe2x88x926 to 10xe2x88x922 moles of total metal contained in the catalyst per liter of reaction medium.
Advantageous results are obtained using quantities of catalyst ranging from 10xe2x88x924 to 10xe2x88x923 moles of total metal contained in the catalyst per liter of reaction medium.
The reaction solvent can consist of water, a C1-C3 alcohol or their mixtures.
Among C1-C3 alcohols, methanol is preferred for the purposes of the invention. Among the mixtures, a mixture of methanol and water with a weight ratio ranging from 50/50 to 99.9/0.1, preferably from 90/10 to 99/1, is preferred.
The acid promoter can be any substance capable of generating H+ hydrogen ions in the liquid reaction medium and is generally selected from inorganic acids such as sulfuric, phosphoric, nitric acids or from organic acids such as sulfonic acids. Sulfuric acid and phosphoric acid are preferred. The concentration of the acid generally ranges from 20 to 1000 mg per kg of solution and preferably from 50 to 500 mg per kg of solution.
The halogenated promoter can be any substance capable of generating halogen ions in the liquid reaction medium. Substances capable of generating bromide ions are preferred. These substances are generally selected from hydrobromic acid and its salts soluble in the reaction medium, for example sodium bromide, potassium bromide, sodium or ammonium bromate. Hydrobromic acid, sodium bromide and potassium bromide are particularly preferred.
The concentration of halogenated promoter generally ranges from 0.1 to 50 mg per kg of solution and preferably from 1 to 10 mg per kg of solution.
The production of hydrogen peroxide is carried out by reacting the oxygen and hydrogen in the reaction medium in the presence of the catalyst and promoters and in the presence of or without an inert gas selected from nitrogen, helium, argon. The gas is preferably nitrogen.
The molar ratio H2/O2 in the feeding ranges from xc2xd to {fraction (1/100)}, preferably from ⅓ to {fraction (1/15)} and the concentration of hydrogen in the gaseous phase in contact with the liquid reaction medium is conveniently maintained at a value lower than 4.5% molar, outside the explosivity limits of the H2/O2/inert gas mixture.
According to an embodiment of the process of the present invention, the reaction can be carried out using air instead of pure oxygen.
The reaction is typically carried out at temperatures ranging from xe2x88x925xc2x0 to 90xc2x0 C., preferably from 2 to 50xc2x0 C. and at a total pressure higher than atmospheric pressure, preferably ranging from 50 to 300 atmospheres.
The process according to the present invention can be carried out batchwise, or, preferably, in continuous using a reactor suitable for the purpose and selected from those described in the art.
Operating under the above conditions, it is possible to produce hydrogen peroxide in safety conditions with a reaction productivity normally ranging from 30 to 200 g of H2O2 (expressed as 100% H2O2) per liter of reaction medium per hour and with a molar selectivity towards the formation of H2O2, referring to the hydrogen used up, ranging from 60% to 90%. The hydrogen peroxide solutions thus obtained can be used directly in oxidation processes which comprise the use of H2O2 without costly intermediate processing, such as acid and solvent removal operations.
The process of the present invention enables the reagents to be transformed into H2O2 with high conversions and selectivities, obtaining H2O2 solutions without acidity or containing only traces of acidity and/or salts.
The following examples, whose sole purpose is to describe the present invention in greater detail, should in no way be considered as limiting its scope.